Uroguanylin, guanylin and bacterial ST peptides are structurally related peptides that bind to a guanylate cyclase receptor and stimulate intracellular production of cyclic guanosine monophosphate (cGMP) (1,6). This results in the activation of the cystic fibrosis transmembrane conductance regulator (CFTR), an apical membrane channel for efflux of chloride from enterocytes lining the intestinal tract (1-6). Activation of CFTR and the subsequent enhancement of transepithelial secretion of chloride lead to stimulation of sodium and water secretion into the intestinal lumen (3). Therefore, by serving as paracrine regulators of CFTR activity, cGMP receptor agonists regulate fluid and electrolyte transport in the GI tract (1-6; U.S. Pat. No. 5,489,670). Thus, the cGMP-mediated activation of CFTR and the downstream signaling plays an important role in normal functioning of gut physiology. Therefore, any abnormality in this process could potentially lead to gastrointestinal disorders such as irritable bowel syndrome, inflammatory bowel disease, excessive acidity and cancer (25, 26).
The process of epithelial renewal involves the proliferation, migration, differentiation, senescence, and eventual loss of GI cells in the lumen (7, 8). The GI mucosa can be divided into three distinct zones based on the proliferation index of epithelial cells. One of these zones, the proliferative zone, consists of undifferentiated stem cells responsible for providing a constant source of new cells. The stem cells migrate upward toward the lumen to which they are extruded. As they migrate, the cells lose their capacity to divide and become differentiated for carrying out specialized functions of the GI mucosa (9). Renewal of GI mucosa is very rapid with complete turnover occurring within a 24-48 hour period (9). During this process mutated and unwanted cells are replenished with new cells. Hence, homeostasis of the GI mucosa is regulated by continual maintenance of the balance between proliferation and apoptotic rates (8).
The rates of cell proliferation and apoptosis in the gut epithelium can be increased or decreased in a wide variety of different circumstances, e.g., in response to physiological stimuli such as aging, inflammatory signals, hormones, peptides, growth factors, chemicals and dietary habits. In addition, an enhanced proliferation rate is frequently associated with a reduction in turnover time and an expansion of the proliferative zone (10). The proliferation index has been observed to be much higher in pathological cases of ulcerative colitis and other GI disorders (11). Thus, intestinal hyperplasia is the major promoter of gastrointestinal inflammation and carcinogenesis.
In addition to a role for uroguanylin and guanylin as modulators of intestinal fluid and ion secretion, these peptides may also be involved in the continual renewal of GI mucosa by maintaining the balance between proliferation and apoptosis in cells lining GI mucosa. Therefore, any disruption in this renewal process, due to reduced production of uroguanylin and/or guanylin can lead to GI inflammation and cancer (25, 26). This is consistent with previously published data in WO 01/25266, which suggest a peptide with the active domain of uroguanylin may function as an inhibitor of polyp development in the colon and may constitute a treatment of colon cancer. However, recent data also suggest that uroguanylin also binds to a currently unknown receptor, which is distinct from GC-C receptor (3,4). Knockout mice lacking this guanylate cyclase receptor show resistance to ST peptides in the intestine, but effects of uroguanylin and ST peptides are not disturbed in the kidney in vivo (3). These results were further supported by the fact that membrane depolarization induced by guanylin was blocked by genistein, a tyrosine kinase inhibitor, whereas hyperpolarization induced by uroguanylin was not effected (12, 13). Thus, it is not clear if the anti-colon cancer and anti-inflammatory activities of uroguanylin and its analogs are mediated through binding to one or both of these receptors.
Irritable bowel syndrome (IBS) and chronic idiopathic constipation are pathological conditions that can cause a great deal of intestinal discomfort and distress but unlike the IBD diseases such as ulcerative colitis and Crohn's disease, IBS does not cause the serious inflammation or changes in bowel tissue and it is not thought to increase the risk of colorectal cancer. In the past, inflammatory bowel disease (IBD), celiac disease and irritable bowel syndrome (IBS) were regarded as completely separate disorders. Now, with the description of inflammation, albeit low-grade, in IBS, and of symptom overlap between IBS and celiac disease, this contention has come under question. Acute bacterial gastroenteritis is the strongest risk factor identified to date for the subsequent development of postinfective irritable bowel syndrome (PI-IBS). Clinical risk factors include prolonged acute illness and the absence of vomiting. A genetically determined susceptibility to inflammatory stimuli may also be a risk factor for irritable bowel syndrome. The underlying pathophysiology indicates increased intestinal permeability and low-grade inflammation, as well as altered motility and visceral sensitivity (27). Thus, IBS is now considered as a low grade IBD
Serotonin (5-hydroxytryptamine [5-HT]) is a key modulator of gut function and is known to play a major role in pathophysiology of IBS. It has been shown that the activity of 5-HT is regulated by cGMP (28). Recent studies have shown measurable improvements in patients with IBS treated with selective serotonin reuptake inhibitors and serotoninergic agents (alosetron, tegaserod) (29, 30). Majority of the serotonin content in the body is found in the gut and not in the central nervous system. This fact raises the question as to whether the modulation of serotonin action in the gut could influence IBS or other functional bowel symptoms. Recently, it has been suggested that mucosal inflammation plays a putative role in the pathophysiology of IBS (31). Therefore, we believe that GC-C agonist might also be useful in treatment of IBS.
Diabetes mellitus (DM) is a group of metabolic diseases characterized by hyperglycemia, resulting from defects in insulin secretion, insulin action, or both. The chronic hyperglycemia of diabetes is associated with long-term damage, dysfunction, and failure of various organs, especially the eyes, kidneys, nerves, heart, and blood vessels. Well-known risk factors of type 2 DM are family history, obesity, age, race, prediabetes [impaired fasting glucose (IFG) and/or impaired glucose tolerance (IGT)], gestational DM, polycystic ovarian syndrome, and. A few previous studies have evaluated possible relationships between insulin resistance and serotonin (32). Investigations on diabetic rats have revealed dysfunctions in serotonin receptors in both the large and the small intestines (33). Moreover, an association between insulin resistance and inflammation has been reported (34). In light of these data, we hypothesize that IBS may be associated with levels of glucose tolerance, resulting in type 2 DM. Therefore, GC-C agonists may also be useful in prevention and control of type 2 DM.
Furthermore, there are numerous investigations that have supported the role of chronic inflammation in the pathogenesis of type 2 DM (35-37). In these studies, it was noted that chronic inflammation might accompany increased levels of C-reactive protein and inflammatory cytokines. Data also indicated a correlation that prediabetes was common in patients with IBS, which suggested that the chronic inflammation process might be responsible for the progression to DM. Prediabetes condition has recently been reported to occur more commonly in the IBS group than in the control group (35). HDL and LDL levels were also found to be higher in the IBS group compared with the control group (35). Because prediabetes is a precursor of type 2 DM, patients with IBS may be considered as a high-risk group for type 2 DM. Hence, treatments for IBS might also prevent and control progression of prediabetic condition to type 2 DM.
Hypercholesterolemia has been recognized as a major risk factor for coronary heart disease (CHD). In clinical trials, reducing serum LDL cholesterol has been demonstrated to decrease the incidence of CHD and to reverse atherosclerotic lesions. Two main classes of clinically useful hypocholesterolemic agents are the 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (e.g., statins) and the bile acid sequestrants. Both induce hepatic LDL receptor activity by increasing hepatic cholesterol demand. Because the major determinant of serum cholesterol level is hepatic LDL receptor activity (38), these agents may share a common mechanism leading to reduction in serum cholesterol.
In the case of bile acid sequestrants such as cholestyramine and colestipol, the mechanism of action seems to be due to inhibition of enterohepatic circulation, the transport of bile acids between liver and intestine. Bile acids are synthesized from cholesterol in the liver and secreted into the bile flow to facilitate the digestion and absorption of lipids, followed by nearly quantitative (˜0.95%) reabsorption from the intestine. The remaining ˜5% of the bile acids enter colon and excreted out. The ileal Na1/bile acid cotransporter (IBAT) maintains the reabsorption of bile acids from the intestine and thus, its inhibitor is expected to exhibit pharmacological effects similar to those of bile acid sequestrants. Bile acids are detergent molecules that facilitate biliary excretion of cholesterol, byproduct of metabolism and xenobiotics, and intestinal absorption of fat and fat-soluble nutrients. When food is ingested, the gallbladder is stimulated to contract resulting in secretion of bile into the lumen of small intestine (duodenum), where it acts as a detergent to form micelles of fat soluble nutrients, dietary cholesterol and lipids. Micelles serve an important function in the digestion and in absorption of fat consisting of mainly dietary triglycerides and cholesterol.
The digestive system is largely responsible for the maintenance of cholesterol balance in the body. Bile salts are produced by enzymatic modification of cholesterol and secreted into intestine. The reabsorption of bile salts from the intestine is very efficient and 95-98% of bile salts are recycled back to liver. Thus, only 2-5% of bile salts escape recycling and are excreted out in feces. This amount of loss of bile salts is replenished quickly in liver through enzymatic conversion from cholesterol. Therefore, inhibition of bile salt from the intestine has been used as an approach to reduce serum cholesterol. Moreover, cholesterol absorption inhibitors also reduce the absorption of dietary cholesterol. Known cholesterol absorption inhibitors are plant sterols and stanols. In addition, inhibitors of ileal Na+/bile acid cotransporter (IBAT) are also used for reducing plasma cholesterol. Plasma cholesterol levels can be reduced through inhibition of cholesterol synthesis as well as through inhibition of ileal absorption of dietary cholesterol and reabsorption of bile salts. Enterohepatic cycling thus has a profound impact on plasma cholesterol and body fat.
Prolonged small intestinal transit, like in patients with chronic constipation, IBS-c, and impaired gallbladder emptying, should hinder enterohepatic cycling, which might be associated with increased levels of plasma cholesterol, triglycerides and lipids. In addition, slowed transit through the distal intestine (ileum, caecum and colon) may also lead to increased conversion of bile acids to deoxycholate, which in itself can slow down small intestinal transit. Absorption of deoxycholic acid from colon occurs only by passive diffusion. When radioactive cholic acid is injected into the colon at a laparotomy most of it was absorbed and resecreted in the bile largely as deoxycholate during the first 24 hours but its absorption from colon continued for several days (39). Moreover, prolonged presence of deoxycholate at higher level in colon can also cause inflammatory diseases and cancer.
Given the prevalence of diseases associated with hypercholesterolemia, obesity and inflammatory conditions, inhibition of ileal absorption of choleaterol and reduction in reabsorption of biles salts from intestine could be highly useful as improve the treatment options for obesity, cardiovascular diseases, diabetes type 2, gallstone and liver diseases.